I think Samurott fits into most team a lot better. Carracosta really got nothing but smash, and even with the smash it's not super fast either. It's certainly workable and probably more destructive once it got the chance to setup, but Samurott is just the safer pick that fits into more teams.

They're both pretty bleh in design though XD Carracosta's pre-evolution is a lot better in the design department XD